1. Field of the Invention
The field to which this invention most nearly relates is measurement and recharging apparatus for condenser ionization chambers.
2. Description of the Prior Art
A condenser ionization chamber comprises an ionization chamber with a capacitor connected in parallel with the electrodes of the chamber. The structure and operation of such chambers are described in detail in many references such as Fundamental Physics of Radiology by Meredith and Massey (1972). Before the chamber is used, a charge is placed on the capacitor. When radiation passes through the ionization chamber, the gas in the chamber is ionized, and the ions are attracted to the electrodes of the chamber by the voltage impressed on them by the parallel connected capacitor. These ions discharge the capacitor, and the amount the capacitor is discharged is a direct measure of the radiation exposure. The concomitant voltage drop on the capacitor is then an indication of the radiation dose received by the chamber.
Prior art measurement apparatus usually measure the voltage on the capacitor before and after exposure to radiation, determine the voltage difference, and then convert the voltage difference into the equivalent charge, based on the value of the capacitor in the chamber. Several different means for making such measurements are known and are described in the above cited reference and in U.S. Pat. No. 3,010,021 to Roesch et al.
There are a number of disadvantages to the prior art measurement systems since they usually require measuring the difference between two relatively large voltages along with the conversion of this voltage to the desired quantity, charge. The conversion process is also subject to errors in the measurement of the value of the capacitor in the ionization chamber. In addition, most prior art devices require separate measuring and recharging operations. Even those devices which do measure and recharge in the same operation do not measure directly the desired quantity, charge.
When a large radiation dose is measured, the capacitor across the ionization chamber is discharged by a large amount, changing the field strength in the chamber significantly. This can cause errors due to recombination loss of ions in the chamber. In addition, the large voltage difference between the terminals on the condenser ionization chamber and the measurement device terminals can cause sparking, introducing measurement errors.
These last two problems can be solved by using a different kind of ionization chamber -- the cable-connected ionization chamber. This type of chamber, shown for example in U.S. Pat. No. 3,614,444 is always connected to the metering circuitry by a cable and always kept fully charged via the cable. This being the case, no capacitor is required in parallel with the ionization chamber. This type of ionization chamber, however, has the significant disadvantage that it is not highly portable since movement of the chamber is limited by the cable.
Condenser ionization chambers are usually quite small since they are often used for personnel radiation monitoring, and a cable-connected ionization chamber is totally unsuitable for such use.